Patent GB-1101495-A (GB)
Title: Improved fluid-flow measuring apparatus
Inventor: ROMANOWSKI ALBERT FRANK
IPC: G01F1/58
Filed: 1965-06-08
Abstract: 1,101,495. Induction flowmeters. BOWSER Inc. June 8, 1965, No. 24157/65. Heading G1N. In an induction-type fluid flowmeter, at least two spaced apart magnetic fields are applied across a composite tube consisting of a short inner insulating tube 26, Fig. 4, and an outer conductive tube 28, and are arranged so that the eddy currents induced in the conductive tube by the magnetic fields substantially cancel one another, the emf. induced in the liquid flowing in the tube being picked up by a terminal 30 situated in one of the magnetic fields. The terminal 30 passes through both tubes, and is insulated from the outer conductive one, a second terminal 38 being connected to the outer tube diametrically opposite the first terminal; connecting leads (44, 46, Fig. 3, not shown) to the terminals are embedded in a longitudinal groove in the outer tube, and leave the tube at a point outside the influence of the magnetic fields. The spaced fields are achieved by placing the tube symmetrically in the air gap of a C-type core (12, Fig. 1, not shown) carrying A. C. windings (16, 18) the tube also passing through an aperture (24) in the limb (22) of the core remote from the air gap. Alternatively a double-C type core may be used Fig. 9 (not shown). The output from the terminals 30, 38 is applied to an amplifier (48, Fig. 5 not shown) driving a transformer (50) and a chopper (58). The armature, (60) of this chopper is operated by an A. C. supply, on lines (78) which also energizes the magnetic field windings (16, 18), the connection to the chopper being made via an adjustable phase shifting circuit (96). The output of the chopper is unidirectional and is fed via a resistor (64) to a smoothing capacitor (68), and the resulting D. C. is fed to the armature (61) of a second chopper which is also operated by the A. C. supply and which feeds a transformer (72) and a servoamplifier (74). The amplifier output is an A. C. signal, the phase of which is proportional to the amplitude of its input and therefore to the amplitude of the output of the first chopper (58). The servoamplifier output energizes one winding of a twophase servomotor (76) which drives a pointer (80) and a moving tap (84) on a potentiometer (86). The potentiometer (86), supplied with a 180‹ shifted signal from the A. C. source, applies a nulling signal via a transformer (92) to back off the output of the tube terminals, the servomotor ceasing to operate when the net input to amplifier (94) is zero. When setting the zero of the apparatus under noflow conditions, a small output occurs at the terminals (36, 38) due to the static liquid in the tubes, this output being phase-displaced from the A. C. supply, and therefore from the output of transformer (92), on account of various inductive and capacitive effects in the tube, (Fig. 4) and consequently the input to the amplifier (48) cannot be completely reduced to zero. The A. C. driving the chopper (58) is therefore phaseadjusted (at 96) so that the armature will rest on the two contacts (54, 56) respectively for a brief period embracing successive zero-cross-over points of the alternating output of the amplifier (48). The chopper (58) output Fig. 6c (not shown) is then smoothed (at 68), giving zero input to the next chopper. When the liquid flows, a substantial signal is developed across the tube terminals, and as the sum of this large signal and the output of the nulling amplifier (92), does not have the same zero-cross-over points as the net signal under no flow conditions, an output Fig. 6d (not shown) appears from chopper (58) and the servomotor operates to move the pointer (80) and to adjust the potentiometer (86) and balance the system as described.